In the automotive industry, as in many other industries, metal parts are manufactured in large volume. Processes for manufacturing the parts may include casting and machining. In the automotive industry in particular, manufacturers prefer that parts used in engine manufacture be within tolerance for long and consistent operation of the engine. A quality metric, quality index, or quality standard may be defined based on the extent to which a part is measured to be within tolerance. A nonconforming part is one that does not meet prescribed quality standards or manufacturing tolerances.
After or during their production, many parts used in engine manufacture are measured to assess their quality. Many manufacturing and production plants use a Coordinate Measuring Machine, or CMM. The CMM is a measuring machine tool that may have three axes with corresponding displacement transducers, a probe head to probe the workpiece in a plurality of spatial directions, a control unit, and a processor with software to perform calculations and present results.
The basic function of coordinate metrology consists of the measurement of the actual shape of a workpiece, its comparison with the desired shape, and the evaluation of the metrological information, such as size, form, location, and orientation. The actual shape of the workpiece is obtained by probing the surface at discrete measuring points. Thus, one step in coordinate metrology is selecting the set of features to be measured, the measuring points. Every measuring point is expressed in terms of its measured coordinates. The practice of metrology using the CMM can be defined as: generating the measured data by probing measuring points of the actual workpiece with the probe in the CMM; calculating the parameters of the workpiece such as size, form, location and orientation (e.g. diameter of a hole, the distance between two holes, the angle of a surface relative to a datum, and so on); and, evaluating the required workpiece features by comparing the calculated parameters with the drawing dimensions and tolerances.
Currently, in standard manufacturing procedure, all features or measuring points of a part are measured. The full measurement of the part requires significant time (generally 30 to 100 minutes). For example, the full measurement of a cylinder head may require 50 measured features and may take more than 100 minutes. Often, human resources are involved in placing the part within and removing the part from the CMM system and monitoring the measurements taken by a CMM system.
Thus, it is desirable to reduce the amount of time required to take the measurements of parts in the automotive and other industries so that fewer CMM systems are needed. Further it is desirable that fewer human operators are needed for the measurement process. It is also desirable to increase the number of parts measured during a given time so that production capacity may be increased while maintaining standards for parts.